Strip plasma for enhancement of interface adhesion between bare Cu leadframe and epoxy molding compound

Delamination between copper (Cu) leadframe and epoxy molding compound (EMC) is a serious quality concern for leadframe packages. An oxide layer on the surface of bare Cu leadframe is a dominant factor effecting delamination between interface of EMC and Cu leadframe. In this paper, an HLQFP package was selected to evaluate the correlation between oxidation of bare Cu leadframe and zero-hour delamination. HLQFP package that applied high thermal enhanced glue material required curing temperature at 250°C, this heat treatment causing leadframe anti-tarnish decomposed before wire bond process. Normally, leadframe anti-tarnish would be gradually decomposed after 200°C. Therefore, leadframe was not protected by anti-tarnish after glue curing. Additionally, Cu wire bond required high bonding temperature at 235°C. Therefore, leadframe oxidation layer thickness would rapidly grow during wire bond process, even worse if there was unpredicted machine stop. The thick leadframe oxidation layer would lead to severe delamination on die-pad or lead of Cu leadframe to EMC interface at zero hour. In this paper, we evaluated strip plasma with argon (Ar) and hydrogen (H₂) gas. Hydrogen gas could reduce the Cu oxide film by observation on the change of the leadframe appearance. Scanning acoustic tomography (SAT) inspection was applied to check if there is any delamination gap in the package. In the meantime, package "button" shear was performed to quantify the adhesion of die-pad to molding compound. Two sets of design of experiment (DOE) were conducted and the results shown strip plasma could resolve die-pad delamination effectively. However, package "button" shear data shown no significant changed on adhesion. Furthermore, transmission electron microscopy (TEM) analysis was conducted to observe oxide layer thickness and delamination interface. The results confirmed strip plasma could reduce Cu oxide to Cu and the delamination occurred at Cu oxide and leadframe Cu strike interface. Time of flight secondary ion mass spectrometry (ToF-SIMS) analysis confirmed strip plasma with low power setting could reduce the risk of cation/anion re-deposition.